1. Field of the Invention
The present invention relates to desalination plants for producing potable water, and particularly to a once-through multi-stage flash (MSF) desalination.
2. Description of the Related Art
Desalination is a growing industry in many parts of the world. Both countries with vast areas of arid land, as well as countries having more diverse climates are increasingly producing fresh water through desalination to meet the demands of growing populations and rising standards of living.
Several desalinating methods for seawater or brine have been developed, and may mainly be classified into two types. The first type is membrane isolation, such as reverse osmosis filtering or electrodialysis. The reverse osmosis method is suitable for desalinating seawater, and the electric dialysis is suitable for treating brine containing lesser quantities of salt.
The second type is thermal separation. The thermal separation techniques include two main categories: the first is evaporation followed by condensation of the formed water vapor; and the second involves freezing followed by melting of the formed water ice crystals. Evaporation followed by condensation is the most common thermal separation process used in desalination, and in nearly all cases, it is coupled with power generation units. The evaporation process may take place over a heat transfer area and is termed as boiling or within the liquid bulk, and is generally defined as “flashing”. The evaporation processes include the multi-stage flash desalination (MSF) process, the multiple effect evaporation (MEE) process, the single effect vapor compression (SEC) process, the humidification-dehumidification (HDH) process, and the use of solar stills. MSF desalination includes several configurations, such as “once-through”, brine recirculation and brine mixing.
The simplest once-through flashing process may occur in a single stage and can be described as follows: Feed seawater (i.e., brine) is first heated as liquid (under pressure) in a heater, known as the top brine heater, using steam from an external source. The feed is prevented from boiling by keeping the pressure throughout the top brine heater above the saturation pressure corresponding to its temperature. The brine is then throttled to a flash chamber, with the chamber being generally referred to as a “flashing stage”. The pressure in the chamber is lower than that corresponding to the brine temperature leaving the heater. The pressure inside the flashing chamber is maintained by a suitable ejection system. As a result of the throttling process, some vapor will flash off the bulk of the brine. The flashed off vapor is then condensed over the outer surface of a tube bundle, referred to as a “condenser”, by heat transfer to a cooling stream flowing inside the tubes. After that, the condensate is collected over collecting trays and withdrawn out of the stage as potable water. The remaining high salinity brine will be withdrawn using a pump, and delivered back to sea. In order to reduce the amount of heating steam needed in the top brine heater, the feed is used as the cooling fluid flowing inside the tubes of the condenser. In this way, the latent heat released from the condensation process of the vapor is utilized in heating the feed (i.e., “regeneration”). Moreover, further performance improvement is achieved by passing the hot high salinity brine to another flashing chamber, or a series of chambers, maintained at lower pressures to form the MSF plant.
The conventional MSF plant consists of a series of stages, where the first stage (the top stage) has the highest temperature, since it is heated by steam from an external source, and is generally referred to as the “top brine heater”. The last flashing stage (the bottom stage), on the other hand, has the lowest temperature, since it exchanges heat with the sea, and is generally referred to as the “condenser”. The intake seawater is introduced into the inside of the condenser tubes of the last flashing stage. The temperature of the feed water determines the temperature of the last stage. The feed is heated by regeneration in the condenser (feed-water-heater) and externally by heating steam in the brine heater. The flashing brine flows counter to the feed flow from the first to the last flashing stage. The heating steam, with a temperature range of approximately 97° to 117° C., drives the flashing process. The heating steam flows on the outside of the brine heater tubes and the feed stream flows on the inside of the tubes. As the heating steam condenses, the feed stream gains the latent heat of condensation and its temperature reaches the desired top brine temperature. This parameter, together with the flashing temperature in the last stage (controlled by the seawater feed temperature), defines the total evaporation range (also called the “flashing range”). The hot brine enters the first flashing stage, through a throttling obstacle, and then a small amount of product vapor is formed. The flashing process reduces the temperature of the unevaporated brine. The temperature reduction across the flashing stages is associated with a drop in the stage pressure, where the highest stage pressure is found in the first stage after the brine heater, and the lowest pressure is that of the last stage. The pressure drop across the stages allows for brine flow without the use of interstage pumping units. In each stage, the flashed off vapor condenses on the outside surface of the condenser tubes. The condensed vapor collects over the distillate trays across the flashing stages to form the final product water, which is withdrawn from the last flashing stage. The condensation process releases the vapor latent heat, which is used to preheat the feed water. As a result, the seawater temperature increases to a higher value. In a once-through MSF, the brine in the last stage is blow back down to the sea.
Additional units in the desalination plant include pretreatment of the feed and intake seawater streams, such as screening, filtration, deaeration, and addition of antiscalant and foaming inhibitors. Other basic units in the system include pumping units for the feed seawater, and brine blowdown. The amount of fresh water produced from MSF plants depends on the evaporation range (i.e., the span along which evaporation takes place). The evaporation range is defined as the difference between the temperature of the top brine heater and that of the last stage. The top brine temperature of the plant depends on the chemical treatment (scale prevention) and is fixed for the plant. The last stage temperature depends on the feed water temperature. High seawater temperature in the summer reduces the evaporation range. It is common for MSF plants to have two operational modes for summer and winter, respectively, to take into consideration changes in the feed temperature due to seasonal climatic variations. An increase by 10% is reported during winter operation, as compared to summer operation, in some desalination plants in Saudi Arabia.
Adding a cooler to the conventional MSF desalination plant in order to reduce the temperature of feed, hence the temperature of the last stage, provides a means to maintain stable operation of the MSF plant independent of the seasonal climatic variations, and increases the evaporation range, and thus the yield of the plant without exceeding the limits set on the highest top brine temperature. Thus, a multi-stage flash desalination plant with a feed cooler solving the aforementioned problems is desired.